A T cell is a type of lymphocyte, or "white blood cell", that mediates the cellular immune response to foreign macromolecules, termed antigens. While T cells are necessary for normal mammalian immune responses, in some instances it is desirable to inhibit their activation: for example, in some autoimmune diseases, the T cells of a subject respond to "self-antigens", i.e., macromolecules produced by the subject, rather than foreign-made macromolecules, and damage the cells and tissues of the subject.
Autoimmune T cell responses are found in subjects having systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), insulin-dependent diabetes and multiple sclerosis (MS) and contribute to the pathophysiology of each.
T cells also cause graft rejection and graft-versus-host disease (GVHD). Graft rejection is caused by an immune response against the transplanted tissues (the graft), which are recognized as "foreign" by T cells of the recipient (host). Graft-versus-host disease is caused by engrafted T cells, which recognize host-made macromolecules as "foreign."